The present invention relates to a synchronous operation control system for use with a numerically controlled machine such as a gear grinding machine, a hobbing machine, or the like in which a tool and a workpiece are required to operate synchronously, and more particularly a synchronous operation control system for monitoring, on a multiplex basis, a grinding tool mounted on a master shaft, a workpiece mounted on a workpiece shaft, and a servomotor coupled to a slave shaft which rotates the workpiece, for their synchronous operation, and for keeping them in an optimally synchronous condition at all times.
There have recently been put to use synchronously operated gear grinding machines in which a grinding tool such as a grinding wheel having a helical ridge on its outer peripheral surface, and a workpiece such as a gear to be ground are held in mesh with each other, and a shaft on which the grinding wheel is mounted and a shaft on which the gear is mounted are synchronously rotated to cause the grinding wheel to grind and finish the gear to desired dimensions.
One such synchronously operated gear grinding machine is disclosed in U.S. Pat. No. 4,631,869 assigned to the present assignee. The disclosed gear grinding machine is constructed as follows: A feed table is disposed on a bed and movable back and forth, and a traverse table is supported on the feed table for displacement in directions normal to the direction of movement of the feed table. A workpiece to be ground is detachably mounted on the traverse table and rotatable by a workpiece spindle motor.
A column is mounted on the bed, and a turntable is rotatably supported on the column and supports a shift table. The shift table holds a wheel spindle unit on which a grinding wheel is detachably supported in mesh with the workpiece. The grinding wheel is rotated to grind the teeth of the workpiece, i.e., a gear.
In order to finish the gear to desired dimensions, the gear is required to be ground while the two shafts are being operated in a synchronous condition, i.e., the ridge on the grinding wheel is being aligned with the groove between teeth on the gear within a predetermined range. If the gear were ground while the shafts are not being synchronously operated, the gear teeth would be curved, the pitch of teeth of the gear would be varied, or other grinding errors would occur.
Some conventional synchronous operation control systems employ electric means for achieving synchronous operation of the shafts in such synchronously operated grinding machines. To obtain highly accurate synchronism between the shafts, a stepping motor is used as an electric means. For high-speed high-torque synchronous operation, a servomotor is employed as the electric means. Where the stepping motor is used, it is quite difficult to attain a desired grinding speed and a high torque. The synchronous operation control system using the servomotor fails to achieve a desired degree of accuracy of synchronous operation. Therefore, it has been difficult to achieve both high accuracy and increased cycle time on a single synchronous operation control system. When a stepping motor is energized, the intervals between pulses for energizing the stepping motor should be as constant as possible to prevent the stepping motor from running out of control. To meet this requirement, synchronous command pulses and differential command pulses must be added and subtracted through highly complex arithmetic operations, and the summation of the synchronous command pulses and the differential command pulses must depend upon a hardware-implemented logic circuit.
To avoid the above drawbacks, there have been proposed synchronous operation control systems for numerically controlled machines which employ a servomotor and process output data from a rotary encoder coupled to the servomotor according to a software program so that high accuracy and increased cycle time can be attained without relying upon a complex pulse logic circuit.
One example of such a synchronous operation control system is disclosed in Japanese Patent Publication No. 59-35729. The disclosed synchronous operation control system includes a detecting means for detecting the angular position of either a hobbing shaft for cutting the teeth of a gear or a motor for rotating the hobbing shaft, and energizes a motor to rotate the gear in synchronism with the hobbing shaft rotating motor based o a positional signal generated by the detecting means.